Most natural gas plants are designed to condition the feed gas to meet pipeline sales gas specification (e.g., requiring specific hydrocarbons dew point and water content), which is typically achieved by extracting propane plus components. The main revenue from the gas plant operation is generated from sales of the condensate components, which are mainly propane, butanes, and heavier hydrocarbons. Hence, most of the plants are configured to maximize propane recovery. In the past, the ethane content in the feed gas was valued only for its heating content, and there were no significant incentives for ethane recovery. However, with increasing demand from petrochemical facilities to use ethane as a feedstock, ethane can be sold at a premium. Gas plants that were designed for the traditional propane recovery are now considering recovering ethane operation. However, retrofitting an existing facility to produce an ethane product is generally difficult and costly.
Numerous separation processes and configurations are known in the art to fractionate the NGL fractions from natural gas. In a typical gas separation process, a high pressure feed gas stream is cooled by heat exchangers, in most cases using propane refrigeration and turbo expansion, with the extent of cooling depending on the richness of the feed gas and desired level of recoveries. As the feed gas is cooled under pressure, the hydrocarbon liquids are condensed and separated from the cooled gas. The liquid is then expanded and fractionated in a distillation column (e.g., deethanizer or demethanizer) to separate the lighter components such as methane, nitrogen and other light components as an overhead vapor from the NGL bottom products.
For example, Rambo et al. describe in U.S. Pat. No. 5,890,378 a system in which the absorber is refluxed, in which the deethanizer condenser provides refluxes for both the absorber and the deethanizer while the cooling duties are supplied by turbo-expansion and propane refrigeration. Here, the absorber and the deethanizer operate at essentially the same pressure. Although Rambo's configuration can often efficiently recover 98% of the C3+ hydrocarbons by additional equipment to generate refluxes, high ethane recovery (e.g. over 80%) becomes difficult, especially when the feed gas pressure is low (e.g., less than 600 psig). High ethane recovery typically requires lowering the absorber pressure, which in turn increases the recompression horsepower requirement. Unfortunately, the lower pressure also increases the CO2 freezing temperature in the demethanizer, particularly when the feed gas contains a significant amount of CO2.
To circumvent at least some of the problems associated with relatively low efficiency and recoveries, Sorensen describes in U.S. Pat. No. 5,953,935 a plant configuration in which the absorber reflux is produced by cooling and Joule-Thomson expansion of a slipstream of feed gas in addition to expansion of another portion of the feed gas. Although Sorensen's configuration may achieve high ethane recoveries, it may only be applicable to very lean gases, while requiring the demethanizer column to operate at a very low pressure, which once more requires additional residue gas recompression horsepower.
In yet other known configurations, high NGL recoveries were attempted with various improved fractionation and reflux configurations. Typical examples are shown in U.S. Pat. No. 4,278,457, and U.S. Pat. No. 4,854,955, to Campbell et al., in U.S. Pat. No. 6,244,070 to Elliott et al., and in U.S. Pat. No. 5,890,377 to Foglietta. While such configurations may provide at least some advantages over prior processes, they are generally intended to operate on a fixed recovery mode, either ethane recovery or propane recovery. Moreover, most of such known configurations require extensive modifications of turbo expanders and changes in operating conditions when the plants are changed from propane recovery to ethane recovery or vice versa. In most instances, ethane recovery is limited to 20% to 40% while higher ethane recovery would require excessive recompression horsepower and would result in a lower propane recovery.
To circumvent at least some of the problems associated with high ethane recovery while maintaining a high propane recovery, a twin reflux process (described in U.S. Pat. No. 7,051,553 to Mak et al.) employs configurations in which a first column receives two reflux streams: one reflux stream comprising a vapor portion of the NGL and the other reflux stream comprising a lean reflux provided by the overhead of the second distillation column. Similarly, U.S. Pat. App. No. 2010/0206003 to Mak et al. describes an improved natural gas liquid recovery method in which residue gas is integrated to the propane recovery design such that it can be used to reflux the demethanizer during high ethane recovery. While these processes can be operated on either propane recovery or ethane recovery, the configurations are generally suitable only for grass root installation and not for retrofit. Moreover, very high ethane recovery (e.g., over 90%) is still not feasible nor economical using such methods. All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
Thus, although various configurations and methods are known to recover natural gas liquids, all or almost all of them suffer from one or more disadvantages. For example, while some known methods and configurations can be employed for both propane recovery and ethane recovery, the capital and operating costs for such plants can be very high and may not be justifiable. On the other hand, retrofitting an existing propane recovery plant for ethane recovery requires significantly less investment. However, retrofitting requires an entirely different approach on plant configuration and operation. Therefore, there is a need to provide methods and configurations for retrofitting a propane recovery plant for ethane recovery, especially where high ethane recovery over 90% is desired.